Visual perception is mediated by complex interactions amongst neurons in the retina, visual cortex, and subcortical brain structures. The importance of vision to humans and other primates is reflected in the enormous percentage of cerebral cortex devoted to processing visual information. Thus, deficits in visual processing are particularly debilitating and arise from abnormalities not only in the eye, but also in cortical circuitry. For example, strabismus or amblyopia during childhood can have long-lasting effects on the cortical circuits that process visual information. There is also evidence that some forms of dyslexia result from central visual system abnormalities. The proposed studies are aimed at understanding the organization and function of neural circuits to, from, and within the primary visual cortex (V1), with the broader objective of understanding how neural circuits mediate visual perception. In particular, these studies aim to identify: 1) the detailed morphological properties of V1 cell types that project to particular extrastriate visual cortical areas and subcortical structures;2) the sources of direct local input to V1 neurons with projections to particular extrastriate visual cortical areas and subcortical structures;3) the retinal ganglion cell types that provide input, via the LGN, to functionally and anatomically distinct compartments in V1;and 4) how the in vivo visual response properties of individual, identified neurons correlate with the connectivity of these same cell types, as revealed by our previous and ongoing in vitro studies. The first 3 goals will be achieved using novel viral circuit tracing tools developed in the principal investigator's laboratory. These tools allow complete filling of identified projection neurons and monosynaptically-restricted transynaptic labeling from specific projection neurons. The last aim is accomplished by recording visual responses of V1 neurons and labeling them with dye to correlate anatomically distinct cell types with function. The proposed studies will allow an unprecedented view of visual cortical circuits - they will reveal the detailed connectivity of neurons in visual cortex and how these circuits relate to the functional properties of the component neurons. Lay summary: Deficits in visual processing are particularly debilitating and arise from abnormalities not only in the eye, but also in cortical circuitry. The proposed studies will reveal the normal organization and function of visual cortical circuits, which is necessary to understand the underlying causes of visual dysfunction.